1. Field of the Invention
The present invention relates to a circuit board for mounting electronic components, and more particularly to a circuit board wherein the connection portions of an electronic component mounted on a base member and the corresponding leads protruding from the base member are electrically connected.
2. Description of the Prior Art
Electronic components which have been highly densified in recent years cannot alone construct many the various electronic apparatuses available today. Therefore, they are typically packaged on a circuit board. To this end, electronic component mounting circuit boards in various forms have heretofore been developed and proposed.
There are, for example, three types of connection for connecting electronic components and terminals to external connections, such as leads, in a circuit board, the first type is, the so-called PGA type wherein a large number of conductor pins planted in a predetermined array are connected with the electronic components through conductor circuits formed on the circuit board The second type is the so-called TAB type wherein parts of the conductor circuits on the circuit board are used as finger leads for directly mounting the electronic components thereon. The third type is the so-called DTP type wherein the leads and the electronic components are connected by wire bonding and then the whole assembly is molded.
Various concrete devices exemplifying, a circuit board of the DIP type, in which a plurality of leads electrically independent of one another are protruded from a base member and in which the respective leads and the connection portions of the electronic components mounted on the base member are electrically connected, have been proposed in the Japanese Patent Application Laid-Open No. 60-194553, etc. Proposed in this laid open application is:
"A hybrid integrated circuit device characterized in that a circuit base plate which is made of a material selected from the group consisting of silicon, polyimide resin, alumina, epoxy resin, and glass epoxy resin is bonded with adhesives to an island portion of a metallic base ribbon, and in that a circuit element is mounted on said circuit base plate and is molded with a resin." The basic construction of prior-art circuit boards for mounting electronic components as represented by the circuit board of such a hybrid integrated circuit device can be chiefly classified into two types as illustrated in FIGS. 1 and 2. The first basic construction includes a metal member (the metallic base ribbon) located at a portion (the island portion) on which the electronic component is mounted and being quite independent from a plurality of other portions that are destined to become leads. The second basic construction is related to the first basic construction and is a construction wherein the metal member located at a portion for mounting the electronic component thereon and the plurality of other portions destined to become leads are all independent of each other and must therefore be connected by wire bonding.
Prior art circuit boards of the type in which the connection portions of the electronic component mounted on the base member are electrically connected with the respective leads must employ one of the basic constructions as stated above because the individual leads need to be electrically independent Therefore the corresponding terminals of the electronic component must be respectively connected to these leads separately and independently. Also for reasons concerning actual manufacture, the aforementioned two basic constructions seem to be adopted. More specifically, the electronic component mounting circuit board as stated above is entirely molded with a resin by so-called transfer molding after the predetermined wire bonding has been performed, in which case the positioning of the respective leads needs to be easy. To this end, as illustrated in FIG. 2, the leads 21 and the metal member located at the portion (island portion 26) for mounting the electronic component thereon are formed as a single metal plate by the use of a frame or the like beforehand, thereby making it possible for the leads and the metal member located at the island portion to be arranged in the same plane.
With the aforementioned basic construction, however, the following problems which must be solved arise in implementing high-density packaging which is particularly required of a variety of electronic component mounting circuit boards in recent years:
(1) The leads and those conductor circuits on the base plate to which the electronic component is connected must be directly connected by the wire bonding. Therefore, electrodes need to be led out to circuit end portions (the outer peripheral parts of the island portion), not only because the entirety after the mounting of the electronic component is molded with the resin, but also because the wire bonding is carried out with short distances. Accordingly, versatility in the design of a circuit becomes very low.
It is also an indispensable condition that the leads be subjected to gold or silver plating for reliably performing the wire bonding. Other than being used as the base of the electronic component, the
(2) metal member located at the island portion namely, the metallic base ribbon located at the island portion, it used merely for forming or reinforcing the electronic component mounting portion, and it cannot be positively utilized as a wiring circuit. Accordingly, the metallic base ribbon located at the island portion is unsuitable for high-density packaging.
(3) In general, an electronic component generates heat when energized, and this heat must be dissipated to the exterior. Nevertheless, in the electronic component mounting circuit board of the prior art described before, the metal member located underneath the electronic component is not joined with other metal portions but is embedded in the base member. Therefore, even though the metal member is in the circuit board and is an excellent in thermal conductor it cannot be positively utilized as a heat radiating member.
(4) Since the island portion for mounting the electronic component thereon and the lead portions to be connected to the exterior are respectively independent and separate, both can be connected only by wire bonding. Therefore, high-density packaging is difficult, and it is also required that bonding wire pieces are molded with the resin so as to enhance reliability.
(5) Even if the electronic component and the circuit board side are connected by solder in some places, the connections of the former to the leads must fundamentally be done by wire bonding. Therefore, a so-called molded electronic component for general purposes is difficult to use.